In the conventional art, as techniques of diagnosing a microstructure change concerning the above-mentioned precipitates, voids, and the like having occurred in the steel material such as stainless steel by means of quantifying the kind, the amount of occurrence, the depth distribution, and the like thereof, techniques have been adopted that employ an electron microscope, ultrasonic, or the like.
The method employing an electron microscope is a destructive technique in which a sample is cut out from the material and then observed by the electron microscope so that the amount of microstructures near the surface is quantified. Nevertheless, when the depth distribution of the microstructures is to be quantified, problems arise like a large number of samples need be cut out.
In contrast, the method employing ultrasonic is a non-destructive technique in which ultrasonic is projected onto a material so that microstructures having occurred in a material are quantified and thereby diagnosis is achieved. In this method, a large number of samples like in the method employing an electron microscope need not be cut out. Thus, this method is preferable.
FIG. 1 schematically shows outlines of the measurement method employing ultrasonic. As shown in FIG. 1, when ultrasonic indicated by a bold rightward arrow are projected into a material, by an ultrasonic transducer indicated by the smaller rectangle, a signal (bottom surface wave) reflected from the material bottom surface indicated by a bold leftward arrow and a signal (backscattered wave) reflected from the inside of the material indicated by a thin arrow can be acquired by the ultrasonic transducer and converted into an ultrasonic wave form.
At that time, when microstructures such as voids, dislocations, and precipitates are present in the material, reflection from the microstructures is generated and added to the backscattered wave so that the backscattered wave intensity increases. On the other hand, the bottom surface wave intensity varies. Thus, when such intensities can be measured, occurrence of the microstructures can be recognized.
In a case of microstructures of approximately several tens of μm or larger, the ultrasonic intensity reflected and scattered directly from the microstructures is sufficiently large. Thus, the amount of variation in the backscattered wave intensity is large and hence, even when a material diagnostic measurement method employing ultrasonic of the conventional art is used, the microstructures of the material can sufficiently be quantified.
Nevertheless, in a case of fine microstructures, specifically, microstructures of approximately several tens of μm or smaller, the amount of variation in the backscattered wave intensity is as small as approximately 1/1000 of the backscattered wave intensity in crystal grains. Thus, scattering by crystal grains becomes dominant. Accordingly, a change in the backscattered wave intensity associated with the occurrence of microstructures is difficult to be identified as a clear signal. Thus, in a material diagnostic measurement employing ultrasonic of the conventional art, a quantification of these fine microstructure defects has been difficult.
Further, when the microstructures have occurred, the depth distribution thereof also need be quantified. Thus, for example, a technique of quantifying also the depth distribution from the obtained ultrasonic data by using wavelet transformation or the like is tried (e.g., Patent Document 1). Nevertheless, owing to large errors, quantification has been difficult.
Thus, various techniques of quantifying fine microstructures by using ultrasonic have been proposed (e.g., Patent Documents 2 to 4).